Despite progress in detection and therapy, breast cancer remains a public health problem of the highest order. There are more than 184,000 cases per year with more than 44,000 deaths from this cancer annually. Notwithstanding improvements in standard and high-dose chemotherapy, prospects for long-term disease-free survival in these patients, as a whole, remain poor. The advent of high dose chemotherapy with stem cell rescue (HDC/PBSCR) has allowed higher and more effective dosing of chemotherapy. In parallel with these advances in chemotherapy, steady improvements in the use of radioactive-labeled anti-tumor antibodies have occurred. This latter modality has shown significant anti-cancer effects, including some patients with stage IV breast cancer. It is the goal of this proposal to combine radioimmunotherapy (with a humanized anti- CEA antibody) either concurrently with or sequentially after high dose chemotherapy in an effort to develop more effective therapy for metastatic breast cancer. This bimodal combination has been found to be well tolerated and very effective in preclinical testing as well as in clinical trials in myeloid leukemias and non-Hodgkin's lymphoma. In the concurrent trial, the chemotherapy will be fixed in type and amount and radioantibody will be given in gradually escalating doses to observe any toxicity as well as to observe anti-tumor effects. In the other 2 trials, radioantibody will be given to patients who have already received HDC/PBSCR. Patients will be stratified based on tumor burden and given either the 90Y-form or the 121-I-form of this humanized monoclonal antibody agent. These initial phase I/II clinical trials will be performed to assess safety, tolerability and efficacy, with an intent to develop this novel approach into effective therapy for testing in larger phase II/III studies.